From BlenderWiki

Decimate Modifier

Mode:Object mode

Panel:Modifiers (Editing context, F9)

This sub-panel appears in the Editingcontext panel group which is accessed using F9 or clicking button in the Buttons window. This sub-panel is part of the Modifiers parent panel. For further information about the common panel components see the modifier stack section.

Description

The Decimate modifier allows you to reduce the vertex/face count of a mesh with minimal shape changes. This is not applicable to meshes which have been created by modeling carefully and economically, where all vertices and faces are necessary to correctly define the shape, but if the mesh is the result of complex modeling, with proportional editing, successive refinements, possibly some conversions from SubSurfed to non-SubSurfed meshes, you might very well end up with meshes where lots of vertices are not really necessary.

The Decimate modifier is a quick and easy way of reducing the polygon count of a mesh non-destructively. This modifier demonstrates of the advantages of a mesh modifier system because it shows how an operation, which is normally permanent and destroys original mesh data, can be done interactively and safely using a modifier.

Unlike the majority of existing modifiers, the Decimate modifier does not allow you to visualize your changes in Edit mode.

The Decimate tool only handles triangles, so each quadrilateral face is implicitly split into two triangles for decimation.

Options

Decimate modifier.

Ratio

The ratio of faces to keep after decimation, from 0.0 (0%, all faces have been completely removed) to 1.0 (100%, mesh is completely intact, except quads have been triangulated).

As the percentage drops from 1.0 to 0.0, the mesh becomes more and more decimated until it no longer visually looks like the original mesh.

Face Count (display only)

This field shows the number of remaining faces as a result of applying the Decimate modifier.

Examples

Simple plane

A simple example is a plane, and a 4x4 undeformed Grid object. Both render exactly the same, but the plane has one face and four vertices, while the grid has nine faces and sixteen vertices, hence lots of unneeded vertices and faces. The Decimate modifier allows you to eliminate these unneeded faces.

Decimated cylinder

We take a simple example of decimating a cylinder using the default of 32 segments. This will generate a cylinder with 96 faces. When the Decimate modifier is applied, the face count goes up! This is because the modifier converts all quadrangles (quads) into triangles (tris) which always increases the face count. Each quad decomposes into two triangles.

The main purpose of the Decimate modifier is to reduce mesh resources through a reduction of vertices and faces, but at the same time maintain the original shape of the object.

In the following pictures, the percentage dropped in each successive image, from 100% to 5% (a ratio of 0.05). Notice that the face count has gone from 128 to 88 at a ratio of 0.6 (60%) and yet the cylinder continues to look very much like a cylinder and we discarded 40 unneeded faces.

Ratio: 1.0 (100%). Faces: 128.

Ratio: 0.8 (80%). Faces: 102.

Ratio: 0.6 (60%). Faces: 88.

Ratio: 0.2 (20%). Faces: 24.

Ratio: 0.1 (10%). Faces: 12.

Ratio: 0.05 (5%). Faces: 6.

As you can see when the ratio has reached 0.1, the cylinder looks more like a cube. And when it has reached 0.05 it doesn’t even look like a cube!

Once you have reached the face count and appearance you were looking for you can Apply the modifier. If you want to convert as many of the tris back to quads to reduce mesh resources further you can switch to Edit mode, select all vertices (A), and hit AltJ.

High resolution landscape

Decimated landscape, top: original; middle: lightly decimated; bottom: heavily decimated. shows a landscape generated via a careful application of the Noise tool (or, nowadays, of the Displace modifier…), on a quite vast grid. On top, the result for the original mesh and below, two different levels of decimation. To the eye the difference is indeed almost unnoticeable, but as the vertex count goes down there is a huge gain.